1. Polyunsaturated Fatty Acid
Polyunsaturated fatty acids (PUFAs) generally refer to fatty acids that consist of 18 to 22 carbon atoms and have a degree of unsaturation of 2 to 6. Specifically, PUFAs range from linoleic acid (18 carbon atoms, degree of unsaturation=2) to docosahexaenoic acid (DHA) (22 carbon atoms, degree of unsaturation=6).
Of different polyunsaturated fatty acids, docosahexaenoic acid (DHA), eicosapentaenoic acid (EPA), α- or γ-linolenic acid, dihomo-γ-lenolenic acid (DGLA), and arachidonic acid are not only known to play important roles as biological compounds, but also have various useful functions (See, for example, Okuyama Hiromi. ed., “Brain Functions and Lipids” (1997) Gakkai Center Kansai; Sato Kiyotaka. ed., “Development of Functional Lipids” (1992); Watanabe Akihiro et al., “Hepatic Diseases and Therapeutic Nutrition” (1992) Dai-ichi Shuppan.). Some are already in use as pharmaceutical agents, food and nutritional products. In general, these polyunsaturated fatty acids occur in the form of glycerides or phospholipids rather than in the form of free acids, and are used as oils containing polyunsaturated fatty acids.
2. Vitamin C
Vitamin C exhibits various physiological activities: it facilitates synthesis of collagen, the depletion of which causes scurvy; it acts as a biological antioxidant and eliminates free radicals generated in a body; and it is involved in the oxidation/reduction of iron ions of cytochrome C. Other important activities of vitamin C include anticancer activity, stimulation of immune functions and suppression of cholesterol synthesis and associating anti-arteriosclerosis activity. The antioxidation activity and the ability to facilitate collagen synthesis, along with the associating anti-photoaging, anti-UV damage and anti-pigmentation activities, make vitamin C a dermatologically important compound suitable for use in cosmetics (See, for example, “Fragrance Journal,” vol. 25, March (1997) Special edition.). Vitamin C is also added in food and cosmetic products as antioxidant.
3. Polyunsaturated Fatty Acids and Pro-Forms of Vitamin C
Despite their wide range of applications, polyunsaturated fatty acids and vitamin C are susceptible to oxidation. For this reason, much effort has been devoted to developing pro-forms of vitamin C (pro-vitamin C), vitamin C derivatives that are converted to vitamin C once inside a body. Among pro-vitamins C thus far produced are 6-O-palmitoyl- and 6-O-stearyl-ascorbic acid esters, which are antioxidants with high solubility in lipid, 2-O-α-glucopyranosyl ascorbic acid having increased stability against oxidation, and those with phosphoric acid or sulfuric acid bound to position 2. Attempts have also been made to make pro-forms by combining a polyunsaturated fatty acid with vitamin C. Of such pro-forms, the ascorbic acid derivatives in which an acyl group originating from a polyunsaturated fatty acid is bound to position 6 via an ester linkage are known to have higher stability against oxidation than the original polyunsaturated fatty acid and are, thus, useful (See, for example, J. Am. Oil Chem. Soc., 78(2001): 823).
Aside from the functions anticipated from the polyunsaturated fatty acid and ascorbic acid, these pro-forms of polyunsaturated fatty acid and vitamin C may exhibit completely new properties. For example, of the ascorbic acid derivatives in which an acyl group originating from a polyunsaturated fatty acid is bound to position 6 via an ester linkage, 6-O-docosahexaenoyl-ascorbate is known to have anti-arrhythmia activity (Japanese Patent Laid-Open Publication No. Hei 10-139664), calcium antagonist activity (International Patent Publication No. WO 94/20092), and anti-allergy activity (Japanese Patent Laid-Open Publication No. Hei 6-122627). Also, 6-O-γ-linoleyl-ascorbate has proven to have an ability to inhibit aldose reductase (U.S. Pat. No. 6,069,168) and is shown to be effective against streptozotocin-induced diabetes model (Diabetologia. 39(1996): 1047).
4. Techniques to Make Polyunsaturated Fatty Acid Into Powder
Oils containing polyunsaturated fatty acids are generally provided in the form of liquid oil or semi-solid (paste). These oils are susceptible to oxidation and are therefore difficult to handle. In one technique to cope with this problem, linoleic acid is mixed with a water-soluble polysaccharide, such as maltodextrin, gum Arabic and soybean polysaccharide, in the presence of various surfactants, and the mixture is spray-dried to form microcapsules (powderization). The resulting linoleic acid powder has higher stability against oxidation than the original linoleic acid (See, for example, J. Agr. Food Chem., 50(2002): 3984, J. Microencapsulation, 19(2002): 181).
Other techniques for forming polyunsaturated fatty acids into a powder composition have been described in several patent applications. For example, Japanese Patent Laid-Open Publication No. Hei 6-228589 describes a technique for providing a powder that can be preserved stably over a prolonged storage period without forming peroxides, can be readily dispersed in water, and is easy-handling. In this technique, a water-soluble excipient comprising a protein or a polysaccharide or an alkaline metal salt thereof is used to form an O/W type emulsion, which is spray-dried. The water-soluble excipient on the surface of the resultant oil powder is modified with an acid or a metal with a valency of 2 into a water-insoluble form, which in turn is dispersed in a solution of water-soluble excipient. The solution is then spray-dried to form a water-soluble film on the surface of the oil powder.
A technique described in Japanese Patent Laid-Open Publication No. Hei 7-313057 uses defatted soybean to serve as an excipient and is particularly effective in improving the stability of polyunsaturated fatty acids against oxidation. Japanese Patent Laid-Open Publication No. Hei 10-99056 describes a technique for providing EPA or DHA-containing powder compositions. According to this technique, an oil-in-water-type emulsion is prepared by using DHA, an oil containing 10 wt % or more of DHA and/or 5 wt % or more of linoleic acid, 2 to 20 wt % of diet fiber as an excipient, 10 to 70 wt % of hydrolysis product of starch or lower sugar product obtained by reductive degradation of starch, and 0.01 to 5 wt % of antioxidant. The resulting emulsion is dried under vacuum.
Japanese Patent Laid-Open Publication No. Hei 9-235584 describes an oil composition obtained by adding 0.0001 to 1 parts by weight of ascorbic acid, along with 0.0001 to 1 parts by weight of at least one selected from citric acid, citrate, malic acid, and malate, to 100 parts by weight of an oil. The resulting oil is emulsified to obtain a W/O type emulsion, which in turn is dissolved in an aqueous phase to form a water-in-oil-type emulsion. This emulsion is then dried to form a powder.
The techniques described above are some of the known techniques for making polyunsaturated fatty acid-containing oils into powder. Each of these techniques, however, concerns a polyunsaturated fatty acid-containing oil, which has different physical properties from the ascorbic acid/polyunsaturated fatty acid ester of the present invention. Therefore, each of the above descriptions offers only limited teaching. The above-described techniques each have a different purpose for making a powder composition: each involves addition of extra additives or other troublesome complicated processes and thus cannot be directly applied to making the ascorbic acid/polyunsaturated fatty acid ester into powder.
Specifically, ascorbic acid esters of polyunsaturated fatty acid of the present invention having 3 or higher degree of unsaturation, including DHA, arachidonic acid and DGLA, do not readily crystallize. Rather, they tend to form a paste, making their handling difficult. Such characteristics of the esters also make them unsuitable for use in pharmaceutical or food products. Although preparations and compositions using these compounds have been described in literatures, such as oral preparations using 6-O-docosahexaenoyl-ascorbate described in Japanese Patent Laid-Open Publication No. Hei 10-139664 and International Patent Publication No. WO 94/20092, in which common excipients for use in oral preparations (e.g., starch, lactose and crystalline cellulose) are mentioned, no specific production process has been described that overcomes the problems caused by the physical properties of the compound. Nor can any specific description be found in U.S. Pat. No. 6,069,168, which describes 6-O-γ-linoleyl-ascorbate, regarding excipients for making tablets containing the compound. Thus, no techniques have thus far been available in this particular technical field for forming a powder composition of polyunsaturated fatty acid/ascorbic acid ester.
Accordingly, it is an objective of the present invention to provide a simple process for making a powder composition comprising a polyunsaturated fatty acid/ascorbic acid ester, which otherwise forms a paste and exhibits undesirable physical properties. It is another objective of the present invention to provide such a powder composition comprising a polyunsaturated fatty acid/ascorbic acid ester.